drm: Check actual format for legacy pageflip.

With modifiers one can actually have different format_info structs
for the same format, which now matters for AMDGPU since we convert
implicit modifiers to explicit modifiers with multiple planes.

I checked other drivers and it doesn't look like they end up triggering
this case so I think this is safe to relax.
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Fixes: 816853f9 ("drm/amd/display: Set new format info for converted metadata.")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210110021142.28221-1-bas@basnieuwenhuizen.nl

drivers/gpu/drm/drm_plane.c

@@ -1163,7 +1163,14 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 	if (ret)
 		goto out;
 
-	if (old_fb->format != fb->format) {
+	/*
+	 * Only check the FOURCC format code, excluding modifiers. This is
+	 * enough for all legacy drivers. Atomic drivers have their own
+	 * checks in their ->atomic_check implementation, which will
+	 * return -EINVAL if any hw or driver constraint is violated due
+	 * to modifier changes.
+	 */
+	if (old_fb->format->format != fb->format->format) {
 		DRM_DEBUG_KMS("Page flip is not allowed to change frame buffer format.\n");
 		ret = -EINVAL;
 		goto out;